.12 THE RUBBER TREE BOOK 



in the soil drowns out the spaces between the soil particles, 

 which would otherwise be partly filled with air. Air is abso- 

 lutely essential for the respiration of roots, and for the healthy 

 existence of the bacteria in the soil. Cultivation is never more 

 essential than in the case of a clayey soil. By means of proper 

 cultivation the minute particles in a clayey soil tend to cling 

 together, thus coarsening the soil and enabling air to penetrate 

 the soil. 



Clay is a very plastic material. It can be puddled, that is 

 to say, so worked up in a wet condition as to be almost non- 

 porous, and puddled clay is therefore largely used for lining 

 drains and the embankments of dams. 



Clay soils under tropical sunshine can be very stiff in hot, 

 dry weather, and planters often prefer to do their forking when 

 the soil is wet. Unless lime is then added, or there is a litter of 

 leaves or other green manure to dig in, working over the soil 

 may not aerate it much. There is no soil which can be more 

 benefited by the addition of vegetable humus in the form of 

 green-manuring than a heavy clay soil. This, or the addition 

 of lime, helps very much in inducing the very minute fine 

 particles of soil which are characteristic of clay soils to 

 cling together in larger masses, allowing for larger air-spaces 

 between. 



Some readers may have a difficulty in understanding how 

 and why the addition of lime, which is recommended as a great 

 assistance in breaking down and rendering available the mineral 

 salts in soil particles, can have the apparently opposite effect 

 of " flocculating," that is to say, of aggregating the fine particles 

 in clay soils and so rendering them easier to work and more 

 permeable to moisture. One of the ablest writers on agri- 

 cultural matters, Dr Hall, late of the Rothamsted Experi- 

 mental Station, deals with this point. The liberty of condensing 

 a few paragraphs out of one of his volumes is taken : 



" The material called clay is characterized by certain 

 properties that are shown when the clay has been puddled, i.e., 

 kneaded when in a moist condition. Water to which a little 

 clay has been added and rubbed up remains turbid for a very 

 long time. Weeks will pass before all the particles settle down 

 to the bottom. Schloesing draws a distinction between the 

 part of the clay (i or 2 per cent, of the whole) which persists 



